1. Field of the Invention
The present invention relates to a display device, and more particularly, to a display device including a driver IC (Integrated Circuit) with a built-in charge pump DC/DC (direct current/direct current) converter.
2. Description of the Related Art
A display device uses a structure in which a glass substrate, a driver IC, and a flexible substrate are joined by thermal pressure bonding method (hereunder, refers to pressure bonding). An example of such a display device is shown in FIGS. 1 and 2. FIG. 1 is a diagram showing the junction of a display panel and a flexible substrate in a conventional display device. In addition, FIG. 2 is a cross-sectional diagram taken along line B-B of FIG. 1.
As shown in FIG. 1, driver IC 101 is mounted on display panel 5. Driver IC 101 is electrically connected to wiring 2 formed on display panel 5 and to connection wiring 3 leading to a driver built-in power supply. In addition, a plurality of FPC (Flexible Printed Circuit) mounting pads 104 is formed on display panel 5. FPC mounting pads 104 are respectively electrically connected to wiring 2 and to connection wiring 3 leading to the driver built-in power supply.
Meanwhile, a plurality of terminals 106 is formed on FPC 7. As for terminals 106 and FPC mounting pads 104 on the side of display panel 5, for example, terminals 106 on both ends are respectively electrically connected by pressure bonding to FPC mounting pads 104 shown by the arrows shown in FIG. 1.
In addition, as shown in FIG. 2, driver IC 101 and FPC 7 are respectively joined to display panel 5 by pressure bonding. Furthermore, bump 10 is formed on driver IC 101. Moreover, ITO (Indium Tin Oxide) film 8 that is a transparent conductive film for preventing oxidation of terminal surfaces is formed on wiring 3. When mounting driver IC 101, driver IC 101 and wiring 3 are electrically connected to each other by performing pressure bonding with anisotropically conductive tape 9 lied between bump 10 and ITO film 8. In a similar manner, terminals 106 on the side of FPC 7 and FPC mounting pads 104 on the side of display panel 5 are electrically connected to each other by performing pressure bonding with anisotropically conductive tape 9 lied between terminals 106 and FPC mounting pads 104.
Meanwhile, for example, as shown in FIG. 1, the widths of the respective terminals/pads of the junction of display panel 5 and FPC 7 were conventionally set uniform due to reasons such as ease of design. In addition, wiring configurations which vary the widths of a portion of the terminals/pads among a group of terminals have been disclosed in, for example, Japanese Utility Model Laid-Open No. 3-125334 and Japanese Patent Laid-Open No. 2007-025710.
FIG. 3 is a diagram showing a junction formed by pressure bonding in a display device disclosed in Japanese Utility Model Laid-Open No. 3-125334. The pressure-bonded section shown in FIG. 3 connects display panel 5 to PWB (Printed Wiring Board) 122 by FPC 7. ITO film 8 is formed on the surfaces of wiring electrode 121a of display panel 5 and wiring electrode 121b of PWB 122. In addition electrodes 121a and 121b and a terminal of FPC 7 are electrically connected to each other by pressure bonding. Note that in FIG. 3, for simplicity, FPC 7 is abbreviated and depicted by dashed lines.
Recently, there has been an increase in display devices provided with a driver integrated circuit (hereinafter referred to as a driver IC) having a built-in charge pump type DC/DC converter. In such a display device, a charge pump in the driver IC charges the voltage from the driver IC via the wirings to a boost capacitor mounted on the FPC. In this case, if the wiring resistance increases due to difference of the wiring material on a glass substrate, the layer constitution of the mounting pads, and the like, a boost voltage loss occurs in association with a voltage drop. As a result, there is a risk that the voltage drop may cause a decline in the display quality of the display device. In the present specification, “resistance” refers to electrical resistance.
In addition, in the conventional display device shown in FIGS. 1 and 2, ITO film 8 is formed, as described above, in order to prevent oxidation of terminal surfaces. In this case, among the resistance of the entire wiring path, contact resistances between terminals and ITO film 8 formed on the surface thereof are relatively high resistance. Now, if the widths of the respective terminals of the junction are uniform as shown in FIGS. 1 and 2, a contact area of the junction is determined by the shapes of the terminals. Therefore, it is difficult to reduce wiring resistance while forming the ITO film.
In addition, in the conventional display device shown in FIGS. 1 and 2, the pitch between the respective terminals (hereinafter also referred to as a terminal interval) is uniform due to the simple design that explains why the terminal width was made uniform. However, with a uniform pitch, since positions of pads on the driver IC side differ from positions of the FPC mounting pads, a wiring path to a boost capacitor on the FPC is determined by terminal positions, resulting in a complicated wiring path. In addition, a uniform terminal pitch also results in a longer wiring path, similarly contributing to an increase in wiring resistance.
With the display device disclosed in Japanese Utility Model Laid-Open No. 3-125334 and shown in FIG. 8, the widths of terminals near both ends of a terminal array are increased easily in comparison to the widths of terminals near the center thereof. However, a wiring which a relatively large current flows, such as a driver IC power supply wiring, is not always disposed at both ends of the terminal array. Therefore, only increasing the widths of terminals near both ends of the terminal array does not necessarily contribute towards reducing in wiring resistance.
With the terminal structure disclosed in Japanese Patent Laid-Open No. 2007-025710, for example, the width of power supply wiring is increased in comparison to signal wiring and a divided terminal structure is adopted for the pressure-bonded section. However, since the pitch between the respective terminals is set approximately uniform, there is a problem in that the wiring path becomes complicated as described above.